The MLE (Modified Ludzak Ettenger) Process is often used to achieve biological nitrogen removal when treating municipal or industrial wastewaters. This MLE process is a two-stage activated sludge process in which the first-stage activated sludge reactor is operated as an Anoxic Carbonaceous Removal/Denitrification (low D.O.=0 to 0.5 mg/L) Reactor and is followed by a second-stage Aerobic Nitrification (high D.O.=1 to 3 mg/L) Reactor.
In order to achieve the desired treatment results, the first-stage Anoxic Reactor must be completely mixed in order to ensure the proper suspension of mixed liquor biological solids within the reactor. Such desirable mixing in the first-stage reactor can be provided by an assortment of mixing apparatuses, such as jet manifolds, floating or fixed mixers, side-entering mixers, and recirculation pumps. Jet manifolds are very effective and economical when disposed at the bottoms of deep tank reactors.
However, this economical use requires the combined operation of a jet recirculation pump and a jet manifold. Such a combination is typically designed to remove a flow of mixed liquor by suction from the anoxic reactor at one location and then to pump the flow back into the anoxic reactor through the jet manifold at another location for dispersion of the flow and of the jet mixing energy within the tank bottom.
Jet recirculation pumps are typically sized by jet equipment manufacturers to provide a very high flow recirculation rate from the tank being mixed back into the same tank being mixed. Typically this jet recirculation pumping rate is much greater than 400% of the throughput flow of the wastewater treatment process.
The MLE Process utilizes a first-stage Anoxic Reactor, which receives the incoming wastewater, and a second-stage Aerobic Reactor which receives ammonia-rich wastewater from the Anoxic Reactor and recycles a portion of its nitrate-rich mixed liquor to the Anoxic Reactor and the remainder to the Clarifier.
The first-stage Anoxic Reactor in the MLE Process provides activated sludge treatment for a dual purpose: 1) to remove carbonaceous BOD by biological synthesis of the incoming wastewater and 2) to denitrify nitrate ions which have been recycled from the second-stage Aerobic Nitrification Reactor. When the nitrate ions are denitrified, oxygen is released and is immediately utilized by the bacteria for synthesis of carbonaceous BOD in the wastewater and for endogenous respiration of biomass in the mixed liquor. Anoxic carbonaceous removal of BOD in the Anoxic Reactor results in the production of ammonia from proteins carried by the incoming wastewater.
Such reactions are known in the art and are illustrated in FIG. 2 of U.S. Pat. No. 6,312,599 B1, issued to John H. Reid. This ammonia, dissolved in the wastewater flowing from the first-stage Anoxic Reactor, is biologically oxidized in the second-stage Aerobic Nitrification Reactor which converts ammonia to nitrite ions and then to nitrate ions in its mixed liquor.
A selected portion of the nitrate ions produced in the second-stage Aerobic Nitrification Reactor is in the mixed liquor which is recycled, according to the MLE process, back to the first-stage Anoxic Reactor in order to serve as an oxygen source for carbonaceous BOD removal in the Anoxic Reactor. This recycling is done by recirculating mixed liquor from the Aerobic Reactor back to the Anoxic Reactor, either by pumping or by gravity flow, depending on the relative elevations of the operating liquid surfaces in the first-stage and second-stage reactors.
The nitrate recycle rate used in the MLE process is typically equal to 200% to more than 400% of the throughput wastewater flow rate. For example, if a thoughput wastewater flow rate and volume of 1.0 million gallons per day (MGD) is required in an MLE process, then the nitrate recycle rate from the second-stage Aerobic Nitrification Reactor (hereinafter Reactor No. 2) back to the first-stage Anoxic Reactor (hereinafter Reactor No. 1), equals 2.0 to more than 4.0 MGD. If a nitrate recycle rate of 200% is used in an MLE process, then approximately 50% of the nitrate nitrogen produced in Reactor No. 2 is recycled back to Reactor No. 1 for removal by biological denitrification.
If a nitrate recycle rate equal to 400% is used in an MLE process, then approximately 67% of the nitrate produced in Reactor N. 2 is recycled back to Reactor No. 1 for removal by biological denitrification. Consequently, as the nitrate recycle rate from Reactor No. 2 back to Reactor No. 1 is increased, the nitrate removal efficiency of the MLE process is also increased.
Unfortunately, the use of a higher nitrate recycle flow rate requires the use of larger nitrate recycle pumps, thereby necessitating greater capital costs for larger pumps and piping and greater operating costs for pumping power.
There is accordingly a need for a more efficient means of transferring large volumes of mixed liquor from one reactor, in the form of a deep tank, for example, to another reactor.
The same need exists in the biological phosphorous (Bio-P) removal process. This biological phosphorous process uses at least three activated-sludge reactors operating in series.
The first-stage reactor is an Anaerobic Reactor, operated with very low nitrate and zero D.O. concentration, that receives both the incoming wastewater and anoxic mixed liquor recycle flow from the second-stage Anoxic Reactor, operated at a very low D.O. concentration, that receives mixed liquor recycle from the downstream third-stage Aerobic Nitrification Reactor. The second-stage and the third-stage reactors of this three-stage process operate in the same manner as the first-stage and the second-stage reactors previously described in the MLE process.
A novel example of the Bio-P process is provided in U.S. Pat. No. 6,312,599 B1 of John H. Reid, wherein flow equalization basins or lagoons are utilized to perform the functions of anaerobic and anoxic tanks.